Alpha-Delta Transitrap Insertion Loss and Return Loss

I use several Alpha-Delta Trasnistrap model E181202 surge protectors
in my station. These are an older model unit that I think is no longer
in production. They are a square metal enclosure about 3.5 inches long,
with an N connector on each end. Here is a network analyzer capture of
the insertion and return loss through 1.3 gHz.

The performance is very good through 2 meters, but the return loss is
less than 20dB above about 168 MHz (though the insertion loss remains
reasonable through the whole range). Note the self-resonance point at
about 1145 MHz.